This invention has to do with the art of cutting by the use of a rotating, axially-moving tool, such as a drill, the art of work holders, and the art of geometrical instruments.
Drilling a straight hole through a precise spot in a workpiece at a precise angle requires careful and rigid positioning of the drill bit and the workpiece. A hand-held power driver drills holes quickly, but the entry point of the bit and the bit direction may meander. A common solution, where it can be used, is a drill press. In a drill press, the spinning chuck assembly is held and moved along a single, usually vertical, axis through bearings that are fixed in a heavy mount. A drill or other bit is mounted in the chuck and is advanced downward into the work by a rack that can move only up and down. Before drilling, the workpiece is carefully positioned beneath the bit and held or clamped in place so that the bit will enter the work precisely where desired and advance through the work in a precise direction relative to the workpiece. The location and direction of the drilling is usually measured and marked in advance on the workpiece so that it may be seen clearly by the operator.
Over many years, various levels of sophistication have been added to the drill press concept, usually to improve speed and efficiency as well as precision in manufacturing operations. These include devices for automated positioning of the work and the employment of multiple cutters moving along multiple axes. Milling machines, for example, have been developed which permit machining coordinates to be entered from digitally-rendered drawings, and the cutters and the workpiece are moved automatically relative to one another along a computed set of coordinates by mechanical positioners until all surfaces and holes are properly cut into the workpiece.
No such sophisticated device is economically available to hand driver users. One simple device, however, for improving the precision of a hand drill is U.S. Pat. No. 4,669,926 to Wilcox, Jr. It describes a guide for a hand drill incorporated into a vise. A tube, with an internal diameter sized to slip fit around a particular drill bit, is positioned by hinged arms over a point centered between the faces of the vise. The vise holds the workpiece, and the tube limits the amount of meandering the bit can do as it is pushed downward into the workpiece.
Geometrical instruments are a separate class of art in which physical representations of geometric entities (points, lines, planes, and solids) are employed to define other geometric entities. (For example, two straightedges may be laid across each other to define a point.) U.S. Pat. No. 5,437,105 to Work exemplifies this type of device. It describes a mechanical device for finding the center of an elongate workpiece having a flat upper surface and straight, but not necessarily parallel, sides. Two straight arms with tracks within them are aligned with the sides of the workpiece. Cross arms which intersect at their centers have ends that ride in the tracks. The intersection of the cross arms will always lie centered between the sides of the workpiece. At that intersection is a scribing pin to mark a point equidistant between the sides of the workpiece.
The above mentioned patent to Wilcox, Jr., combines the art of geometrical instruments with those of cutting and work holding. It allows the user to drill a fairly precisely located hole through the center of a workpiece held by a vise, but being incorporated into the vise, it does not adequately provide for portability. It also relies on a cylindrical drill guide, a tube having a diameter matched to the drill bit being used with it. Such a guide allows some play in the position of the drill bit tip and the angle of drilling. Moreover, a separate drill guide must be used with each different size of drill bit, and such guides may not be used with bits that have an end diameter larger than the shank.
There is also a need for a device, capable of holding both the work and the driver, which will also permit precise drilling at an angle other than vertical.
Another need not met by the prior art is to provide a precise guide for a portable power driver that can be moved around on a large surface with little effort.